Seismic structures of Zhenbei and Huangyan seamounts and their postspreading volcanic evolution

Abstract

The Zhenbei and Huangyan seamounts located close to the extinct spreading ridge (ESR) in the South China Sea (SCS) have provided fundamental insight into postspreading magmatism. Long debates on the source and evolutional process of the postspreading seamounts are due to the lack of detailed structural crustal constraints. Here, we present three-dimensional (3D) crustal thickness and the Moho geometry of the Zhenbei and Huangyan seamounts and their surrounding area by forwarding and interpolating P-wave forward velocity structures along 14 seismic refraction and wide-angle profiles. We analyzed the variation in crustal velocity and thickness from the waning stage of seafloor spreading to the postspreading magmatism stage. Results show that the original oceanic crust, with an average thickness of 4.5 ± 0.7 km, is characterized by thin lower crust, thick upper crust, and low crustal velocity, compared with Atlantic oceanic crust, reflecting reduced magma supply and intense tectonic fracturing. Both the Zhenbei and Huangyan seamounts show high extrusion to intrusion ratio (2.6 and 2.7) and low P-wave velocities in the upper crust, indicating that extrusive processes are dominant during the postspreading volcanism in the SCS. The total magma volumes for building Zhenbei and Huangyan seamounts are estimated to be 4995 km3 and 3674 km3, respectively, with a related volume flux of ∼0.032 m3/s and ∼ 0.023 m3/s. These values are larger than those of Longnan seamount (2884 km3, 0.018 m3/s) but smaller than those of plume-derived seamounts. The relationship between average lower crustal velocity and crustal thickness also indicates that the postspreading magma may not be caused by the higher degree of mantle melting driven by elevated temperature, but by multiple lower degree of melting caused partly by the enriched mantle.